In commercial communication systems, radio frequency interference (RFI) is a well-known phenomenon, particularly in urban areas where wireless coverage is everywhere. Most radio frequencies (RF) fall into the electromagnetic spectrum, ranging from above audio to infrared bands.
In brief, interference comes from an outside source that is external to the preferred signal path and generates unwanted artifacts in the preferred signal. One common source is co-channel interference (CCI) which occurs when two different radio transmitters use the same frequency. In a cellular network, for example, adjacent base stations are assigned to different frequencies. Since there are a finite number of frequencies available, receivers may occasionally be in range of two different base stations using the same frequency, leading to deterioration in receiver performance. A similar problem can happen with radio and television stations, or any system transmitting radio frequencies. In this case, the interference is referred to as RFI (radio-frequency interference).
Another source of interference is adjacent channel interference (ACI). This occurs when extraneous power from an adjacent channel, or frequency, is detected by a receiver. The extraneous power emitted by a transmitter is typically called adjacent-channel leakage and is a function of radio frequency (RF) filters.
A third source of interference is co-site interference. This results from an imperfect receiver that allows nearby frequency signals to leak into its pass band. This type of interference is especially challenging in situations where an adjacent channel is transmitting in close range to a receiver that is trying to receive a weak signal.
It is difficult to avoid RFIs. Possible sources include intentional transmissions, such as those from radio and TV stations or cell phones, as well as unintended radiation from such sources as power lines, appliances and other wireless devices.
Typically, receivers are designed with various filters to improve their ability to pick up only the preferred signal, however integration of a number of standards and frequency bands in transceiver systems impose tough challenges on the receiver design. The receiver chain can be desensitized by large sources of interference, or signal blockers, in a tough signal interference environment. This requires that receivers implement a blocking functionality to remove the RFI with maximum interference power at certain frequency offsets. The blocking function also has to have the flexibility to counter blockers at different frequencies. One traditional way to do this is to employ external passive surface-acoustic wave (SAW) filters to attenuate these blockers to a level that can be handled by the receiver chain. However, practically speaking, this is nearly impossible for integrated multi-mode, multi-standard transceivers since the SAW device not only consumes large amount of printed circuit board (PCB) area thus increasing the handset size, but it also increases the cost. In addition, it complicates the integration and manufacturability of the transceiver front-end.
Another kind of filter used in receivers is a notch filter. This is a band-stop filter with a narrow stopband that is used to cancel portions of a received signal in the stopband. Most current notch cancellation techniques are built, for example, upon feedback or feed-forward (FF) notch cancellation schemes, which are usually narrowband, and difficult to work with, with limited notch cancellation performances. Some wideband notch cancellation employ nonlinear transmission lines (NLTL) as tuning elements. However, these encounter additional spurious response and linearity problems.
Prior art feed forward (FF) interference cancellation techniques experience a number of issues including, for example, the fact that they are difficult to perform at higher microwave/mm-wave frequencies and they are used for single signal cancellation only. Since most FF interference cancellation techniques are aimed at reducing inter-modulation products (IMs), they use one or multiple control loops to reject blockers and loop selectivity depends on open loop gain, not on gain match of two paths.
Therefore, an interference cancellation scheme is needed that addresses with prior art devices using FF and I/Q mixers, for example, asymmetric closed loop transfer function, I/Q gain phase mismatch, and filter core excess noise.